Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art.
One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)3, which has the following structure:

In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.
As used herein, a small area pixel is “equivalent” to a large area panel if (1) the organic stack of the small area pixel consists essentially of the organic layers of the organic stack of the large area panel; (2) the organic stack of the small area pixel is structurally equivalent to the organic stack of the large area panel; and/or (3) the organic stack of the small area pixel is functionally equivalent to the organic stack of the large area panel.
As used herein, the organic stack of a small area pixel “consists essentially of” the organic layers of the organic stack of a large area panel if the organic stack of the small area pixel is expected to have a similar JVL characteristic (where “J” is current density and “V” is voltage and “L” is luminance) as the organic stack of the large area panel. That is, the organic stack of the small area pixel will perform in the same way as the organic stack in the large area pixel. By using this terminology, it is intended to encompass a situation where devices are not exactly identical, but the differences comprise, for instance, only a slight change to the thickness of a layer; a slight modification to a concentration of one of the layers; a material substitution with a material known to behave in the same way, and/or other small modifications such that a person of ordinary skill in the art would understand that the devices would function the same way for purposes of lifetime testing. These situations, and other differences that do not materially affect the characteristics and function of the device, are intended to be covered by this language.
As used herein, a first organic stack is “structurally equivalent” to a second organic stack if the first organic stack comprises materials that are the same as the second organic stack, and the thickness and concentrations levels of these materials (while not necessarily precisely identical) are within experimental error. For instance, the thickness and concentrations of each of the layers of the first organic stack may be within 5% of the corresponding layers in the second organic stack.
As used herein, a first organic stack is “functionally equivalent” to a second organic stack if the first organic stack comprises the same layers as the second organic stack, with only variations that do not significantly affect the JVL characteristics of the organic stack. The variations may be in any form, by way of example, differences in thickness, concentrations, and/or material. If one of skill in the art believes that lifetime data from one device can reasonably be used to predict the lifetime of another device that is expected to have a similar lifetime, the devices are “functionally equivalent.”
More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.
In large area OLED light panels, potential drops due to significant electrode resistances can cause luminance non-uniformity and reduce device efficacy. One method used to reduce potential drops is to introduce highly conductive bus lines. Bus lines are typically designed to deliver current from the electrode contacts and to distribute current evenly across the OLED light panel. Current distribution is then dependent on bus line resistance, electrode resistance, active area and the particular JVL characteristics of the OLED stack. Bus line resistance is determined by the resistivity of the bus line material and the geometry of the bus line, including thickness, length and width. In principle, the resistance of the bus lines could be reduced by using a material with lower resistivity (such as gold, silver, aluminum or copper) or increasing the height of the bus line. However, in practice, there is a finite height at which it is practical to deposit a bus line—any higher than, and it becomes difficult to dispose uniform thin films over the bus lines. Further reduction in the resistance is then typically achieved by increasing the width of the bus lines.
One drawback of using bus lines in OLEDs is that there is usually no light emission from the organic materials disposed over the bus lines. This means that the greater the area of the bus lines, the smaller the area that is available for light emission. When characterizing large area OLED light panels, a critical parameter is luminous emittance in units of lm/m2, which expresses total light output delivered per unit area from the panel. For an approximately Lambertian emitter, luminous emittance (lm/m2)=π×luminance(cd/m2)×Fill Factor, where Fill. Factor is the percentage of the OLED light panel area for which recombination and light emission is enabled. The lower the total area of the bus lines, the greater the Fill Factor and the lower the luminance that is then needed to deliver the same luminous emittance. This requirement for lower luminance, leads to improved lifetime and efficacy for OLED light panels with higher Fill Factor.